Occasional Papers of the Museum of Zoology 55 



greatest diameter, 1.30 mm.; lesser diameter, 1.19 mm. ; height 

 aperture, .47 mm. ; width of aperture, .47 mm. ; greatest diam- 

 eter of umbilicus, .43 mm. 



This minute species appears from its shell characters to be 

 most closely related to Thysanophora tatei Pilsbry, and so 

 belongs within the limits of the genus as at present constituted. 

 It differs from that species by its much smaller size, by the 

 gradually increasing whorls, and by its fine and regular striatu- 

 lations. Under high magnification (700 diameters) all shells 

 show some structure, which is perhaps caused by the edges of 

 the crystals of which it is composed, but these are the most 

 regular that I have ever examined. They are very much more 

 regular and delicate than those of the Striatura-group of the 

 Zonitidae. 



In order to differentiate this species more exactly, and to 

 show its relations to the other Mexican and Central American 

 forms, the following key is presented, which includes all of 

 the species usually placed in Thysanophora, which have been 

 listed from that district. I have examined under rather high 

 magnification (at least 250 diameters) all of the forms included, 

 with the exception of H. guatemalensis C. and F. and T. tiir- 

 blnclla (Mo.). The position of these two forms in this key 

 is doubtful, as their descriptions do not accurately describe 

 the shell sculpture. 



A. Apical whorls spirally striate, without definite markings (Radio- 

 discus?). 1.84; 63; 3^.1 coloba Pilsbry 



AA. Apical whorls with irregular punctations, somewhat radially 

 arranged. Lower whorls also with irregular punctations, with 



1 The numbers for each species indicate in the order given : first, 

 the greatest diameter in millimeters; second, the height-index in per- 

 centages, which is taken as the height divided by the greatest diameter ; 

 and last the number of whorls. The greatest diameter is taken as a 

 basis for the index, as this dimension is usually most accurately deter- 

 mined in small species. 



